Torque calculation, is this correct?

There is a torque arm of 15" with a force of 6000 lbs perpendicular to the axis of rotation. So that calculates to 7500 ft/lbs (6000*15/12). So on the same axis, there is a torque arm of 5" the must oppose this force equally, I need that force required. I calculated 18000 lbs of opposing force with 7500 ft/lbs at the axis (7500/5*12).

Is this valid?

No its not a homework question :) I need to know how powerful a single disc brake on a truck is.